In hybrid vehicles having an operating voltage which is higher than 60 V DC or 25 V AC, safety standards such as ISO 6469-3 require monitoring of the insulation resistance during operation if the traction network is set up electrically isolated from the 14-V electrical system of the hybrid vehicle.
In the presence of an electrical isolation, a single insulation fault does not yet pose any danger of an electric shock to the passengers. In this case, the safety standards ISO 6469-3 and SAE J 2344 therefore allow continued operation of the high voltage system of the vehicle, and thus the electric drive, with the condition that the insulation fault is signaled to the driver visually or acoustically, and that when the vehicle is shut off, a restart may occur only if forced. If a second insulation fault occurs, the safety standards require that the high voltage system of the vehicle is automatically shut down immediately. The insulation fault is ascertained by calculating the insulation resistance from the measured voltage values and the value of the load resistance, and by analyzing the calculated value with the aid of a microcomputer.
A circuit system for determining and displaying the insulation resistance of an accumulator battery is described in German Patent Application No. DE 196 18 897 B4. This determination of the insulation resistance is initialized by operating the ignition key of a motor vehicle and, moreover, also periodically during the driving operation of the motor vehicle. The insulation resistance is determined during isolated operation by measuring an open-circuit voltage and a load voltage. During the measuring process, a series circuit of two capacitors is connected to the terminals of the accumulator battery to store the voltage values obtained during the measurement. The joint point between the two capacitors is connected to a bleed resistor. Furthermore, a switching arrangement is provided, via which measuring amplifiers are connectable to the battery terminals with respect to ground for a predetermined period of time for measuring the partial voltage arising at the first capacitor and the partial voltage arising at the second capacitor. In addition, a switching arrangement is provided, via which the battery terminal having the higher partial voltage may be loaded with a load resistor for measuring the load voltage. Thus, the open-circuit voltage is measurable when the battery terminals are not loaded with a load resistor, and subsequently, the load voltage is measurable after the predefined setting time, when the selected battery terminal is loaded with the load resistor.
A vehicle having an electrical network supplied through fuel cells or batteries is described in German Patent Application No. DE 195 03 749 C1, which is designed as an IT network. Here, the consumers linked to the load current circuit are electrically connected to the car body in a low-resistance manner. Furthermore, an insulation monitoring system including a measuring bridge compensating stage and a measuring-signal-processing buffer amplifier stage is embedded in between the load current circuit and the car body. Furthermore, a control unit is provided, which triggers a pre-alert through a visual or an acoustic warning signal in the case of minor deviations of the insulation resistance from a first threshold value, and which in turn triggers a main alarm and initiates a shut-down of the system in the case of a major deviation.